#include <stdio.h>

/* 你编写的函数将嵌入在这里 */
double Arithmetic(double start, double ratio, int index)
{
    double sum = 0;
    sum = start + ratio * index;
    return sum;
}

int main()
{
    double h, r; // h首项，r公差
    int n;       // n通项索引
    scanf("%lf%lf%d", &h, &r, &n);
    printf("%G\n", Arithmetic(h, r, n));
    return 0;
}
